Technical Field
This Patent Document relates generally to direct-conversion wireless transmitter design, including IQ mismatch compensation.
Related Art
In wireless transceivers, direct conversion can be used for the transmitter (TX) and/or receiver (RX). Direct conversion (zero/low IF) wireless architectures use IQ modulation/demodulation and direct upconversion/downconversion to/from RF, eliminating conversion at an intermediate frequency (IF).
Direct conversion architectures commonly use quadrature (IQ) signal conversion and digital filtering. To meet requirements on out-of-band emissions, direct conversion transmitter designs commonly use digital compensation for TX non-linearities and IQ mismatch (mismatch/imbalance between I and Q signal paths).
TX non-linearities can be compensated by digital pre-distortion (DPD). IQ mismatch is compensated by digital filtering (IQ mismatch compensation or QMC). A feedback receiver (FBRX) is used to capture data required for such compensation.
IQ mismatch generates an image at frequencies reflected about the LO (local oscillator) frequency, which can appear in frequency bands outside the channel reserved for the TX (direct) signal. QMC is used to meet spectral emissions mask requirements for out-of-band interference, such as ACLR (adjacent channel leakage ratio) and ACPR (adjacent channel power ratio).
Various approaches to adapting TX QMC filter coefficients either make assumptions about the IQ mismatch, or restrict the form of the transmitted signal band. For example, TX QMC filter coefficients can be adapted assuming the TX IQ mismatch is frequency-independent, or that it does not drift with temperature. The frequency-independent assumption is not satisfied for transmitters that need to handle broadband signals, such as multi-carrier LTE deployments. Solutions that assume the mismatch does not drift (for example, due to ambient temperature control) can use a one-time calibration with a broadband training signal during system power up.
Other approaches are able to adapt the TX IQ mismatch over a wide bandwidth, and track temperature variations as long as the signal spectrum is restricted to a symmetric band. These approaches require a single carrier or regularly spaced channels.